1. Field of the Invention
This invention relates to vapor-compression evaporative cooling systems that use water as a refrigerant in an open system, and in particular, to vapor-compression evaporative cooling systems capable of processing large volumetric flow rates of water vapor and removing noncondensibles from the system and to methods using such systems. This invention also relates to low-friction, positive-displacement compressors useful in such cooling systems and to means for removing noncondensibles from such cooling systems.
2. Description of the Background
Conventional vapor-compression air conditioning systems employ a working fluid such as chlorofluorocarbons (CFCs). Liquid CFC is introduced into a low-pressure heat exchanger where it absorbs heat at a low temperature and vaporizes. A compressor repressurizes the vapors that are introduced to a high-pressure heat exchanger where heat is rejected to the environment and the vapors condense. The condensate is reintroduced into the low-pressure heat exchanger, thus completing the cycle.
The use of CFCs raises two important environmental concerns. First, CFCs are stable enough to enter the stratosphere where they decompose to chlorine free radicals that catalyze the destruction of ozone. This is unfortunate because ozone absorbs ultraviolet radiation which damages DNA in plants and animals. Second, CFCs absorb infrared radiation which contributes to global warming.
Because CFCs cannot be released into the environment, they must be contained within the air conditioning system. The evaporator and condenser heat exchangers have a sizable temperature difference between the ambient environment and the working fluid (about 10 to 15° C.) which greatly reduces the Carnot efficiency. Further limiting the efficiency is the fact that the condenser rejects heat at the dry-bulb temperature. The wet-bulb temperature is generally about 5–30° C. less than the dry-bulb temperature. Thus, if heat were rejected at the wet-bulb temperature, the Carnot efficiency could be improved even more.
In addition, compressors used in conventional systems typically have compressing components that are in direct contact with each other. The close fit between components has heretofore been necessary to prevent blow-by of high-pressure compressed vapors. However, the friction resulting from the close contact between components reduces efficiency, creates heat and causes wear on the components.
Although the use of water in place of CFCs as the air-conditioning working fluid has been considered, proposed systems have been generally unworkable because the vapor density is very low requiring large volumes of water vapor to be compressed.
One study by the Thermal Storage Applications Research Center of the University of Wisconsin, The Use of Water as a Refrigerant, Report No. TSARC 92-1, Mar. 1992, studied the use of water as a refrigerant. This study concluded that for water-based air conditioning, positive displacement compressors are not suitable for use in such systems. Rather, only dynamic compressors are suitable.
Although “swamp cooler” air conditioners are employed in arid regions of the United States that have low wet-bulb temperatures, they have limited usefulness. In swamp coolers, ambient air is contacted with water which evaporates and cools the air. No external power is required other than for air-handling blowers. Unfortunately, these simple devices are restricted to regions of low humidity (e.g., Arizona, New Mexico) and are not suitable for many regions of the world. Further, although the air is cooler, it has increased humidity which can make the air feel “clammy.”